Typically, a wire-cutting electroerosion machining operation commences at a predetermined start position on a lateral surface on the workpiece but, quite often, also with a starting hole preformed at the preselected position in the workpiece securely mounted on a worktable. The latter is situated in a predetermined plane, X-Y, and is movable by a pair of drive motors along one and the other coordinate in the two-dimensional coordinate system, i.e. in the directions of the X-axis and Y-axis. The positioning of the workpiece relative to the wire electrode should thus occur initially by driving these motors to precisely locate and align the predetermined start position of the workpiece or the preformed starting hole therein with the preset straight line path defined between the aforesaid machining guide members and then loading the wire electrode on the machining guide members or threading the wire electrode through the precision located starting hole of the workpiece. The preset straight line path lies transversely to the aforesaid X-Y plane, i.e. the surface of the workpiece. The threaded or loaded wire electrode must be precisely in alignment with the straight path through the hole and thus is kept taut between the machining guides all the way from the supply side to the collection side. To this end, a traction drive and a brake means are provided at the takeup side and supply side in the wire travel path, respectively. Only when the foregoing positioning step is completed, may the desired wire-cutting electroerosion machining operation be initiated by (a) permitting the threaded or positioned wire electrode to be continuously and axially transported, the control of the traction drive and brake means, respectively (b) supplying the machining region of the workpiece and the wire electrode with the machining liquid and (c) applying the electroerosion machining current between the wire electrode and the workpiece as mentioned above. Then the aforementioned relative machining displacement between the wire electrode and the workpiece along a prescribed contouring feed path is effected controlledly, typically under commands of a preprogrammed numerical control (NC) equipment. In the continuous path of travel of the wire electrode between the supply and collection sides, a number of rollers are typically arranged to allow the wire electrode to be smoothly displaced axially along the path. Furthermore, each of the wire supply and collector or takeup means is typically constituted by a rotating reel.
It has been observed that the axially displaced wire electrode is subject to irregular stresses along such a continuous path of travel and hence tends to deform irregularly and it has long been recognized that a sufficient tension should be applied by the traction drive and brake means respectively to the traveling wire electrode in order to minimize the variation of deformation thereof throughout the given machining operation because any variation tends to cause an undesirable deflection or oscillation of the wire electrode between the machining guides, which in turn causes serious machining inaccuracy. Thus, it has been assumed that solely mounting the continuous wire electrode on the electrode support and guidance system so that it is tightly stretched under a given sufficient tension all the way from the supply means to the takeup means and then driven along the path in the wire positioning step provides a precise positioning of the wire electrode relative to the workpiece during the course of a given electroerosive wire-cutting operation.
The inventor has now discovered that the conventional technique of positioning the wire electrode relative to the workpiece required preparatory to the given wire-cutting electroerosion machining operation is not satisfactory and is even defective, and is indeed one of the most significant causes by virtue of which the conventional wire-cut contour has a limited machining accuracy. It has been found that the physical characteristics of the machining guides and even further of the other support members in the total wire guidance and support system are largely affected by the wire electrode placed in the electroerosion machining system. Accordingly, a significant change in the wire guiding characteristic and position of the system is created between the state when the wire is not energized with the electroerosion machining current and the state when the wire is energized. The change occurs primarily due to heat developed through the wire electrode engaged in the electroerosion machining action, thus due to heat developed by the electrical discharges and/or high-amperage electrical machining current.